Computer Hardware and Logic Circuits: Computer Science 9618 (Cambridge International AS & A Level)
Syllabus 3.1, 3.2 · Strand 3 Computer Systems Architecture
- Questions
- 10
- Total marks
- 51
- Tier mix
- 10 Core
0 of 10 questions completed
Syllabus coverage
- 3.1 6 questions completed
- 3.2 4 questions completed
Every computer system needs a way to take data in, act on it, and store it, and this topic (syllabus ref 3.1–3.2) covers both the physical devices that do this and the logic circuits underneath them. On the devices side, expect input, output, primary memory and secondary storage, plus embedded systems built into everyday products and monitoring/control systems that read sensors (temperature, pressure, infra-red, sound) and drive actuators, often using feedback. Memory itself splits into RAM (volatile, read/write) and ROM (non-volatile) families, with SRAM and DRAM trading speed against cost and density for different jobs.
At the logic level, six gates (NOT, AND, OR, NAND, NOR, XOR) are the building blocks of every digital circuit. Each has a defining truth table, and the three representations of a problem (a written statement, a logic circuit diagram, and a logic (Boolean) expression) can each be derived from either of the others. Being fluent in moving between these three forms, and reading off outputs for every input combination, is the core exam skill here.
The exam-style problems below are original and each comes with a full worked solution.
Question 1
A logic gate has two inputs, P and Q, and one output, R. The truth table for the gate is shown below.
| P | Q | R |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Which logic gate has this truth table?
Question 2
Which hardware component is primarily responsible for converting digital audio data stored in a computer into an analogue electrical signal that can be sent to speakers or headphones?
Question 3
A small games design studio is buying a new desktop computer that will be used mainly for sculpting and rendering detailed 3D game scenery.
(a) State one suitable input device for a designer to use when sculpting and positioning 3D objects on screen, and one suitable output device for viewing the finished 3D graphics in fine detail. [2]
(b) Two of the buses connecting the CPU to the rest of the computer are the address bus and the data bus. State what is transferred along each of these two buses. [2]
(c) Identify one hardware component, other than the CPU, RAM and the buses already mentioned, that is particularly important for this studio's use of the computer, and explain the role it plays. [2]
Question 4
A freelance photographer needs storage for two different purposes:
- while editing a shoot, thousands of RAW photo files must be stored so that they can be opened and saved repeatedly, as quickly as possible;
- once a project is finished, a single archive copy must be written once and then kept off-line, disconnected from the computer, in case the working copy is ever lost or damaged.
(a) Explain how data is physically stored on a magnetic hard disk drive (HDD). [2]
(b) Explain how data is physically stored on an optical disc, such as a DVD. [2]
(c) Explain how data is physically stored in solid-state storage, such as an SSD or a USB flash drive. [2]
(d) State which of the three storage types described in (a) to (c) is most suitable for the photographer's off-line archive copy, and justify your answer. [2]
Question 5
A logic circuit has three inputs, A, B and C, and one output, X. It is built from three gates, connected as follows:
- Gate 1 is a two-input NAND gate, with inputs A and B. Its output is called N1.
- Gate 2 is a single-input NOT gate, with input C. Its output is called N2.
- Gate 3 is a two-input XOR gate, with inputs N1 and N2. Its output is the output of the whole circuit, X.
Write Boolean expressions in plain text: use . for AND, + for OR, and NOT immediately
before a term for NOT (for example, NOT (A.B)).
(a) Describe how the symbol for a NAND gate differs from the symbol for an AND gate. [1]
(b) Write the Boolean expression for X in terms of A, B and C. [2]
(c) Complete the truth table for this circuit for all eight combinations of A, B and C, showing the intermediate outputs N1 and N2 as well as the final output X. [4]
(d) A colleague claims that only inputs A and C affect the output X, and that input B has no effect at all. Using two rows of your completed truth table that have the same values of A and C but different values of B, show that the colleague is incorrect. [2]
Question 6
Which of the following correctly describes a genuine difference between static RAM (SRAM) and dynamic RAM (DRAM)?
Question 7
A digital kitchen scale contains a small built-in computer system. It can only weigh items placed on it, display the weight on a small screen, and reset ("tare") the display to zero. It cannot run any other software, and it has no keyboard, mouse or general-purpose operating system.
(a) Explain what is meant by the term "embedded system", using the kitchen scale as an example. [2]
(b) State two features of the kitchen scale, described above, that show its built-in computer system is dedicated to a single task rather than being a general-purpose computer. [2]
(c) The scale's control program is stored permanently on a ROM chip inside the device, rather than on secondary storage such as a hard disk. Explain one reason why ROM is a suitable choice for storing this program. [2]
Question 8
A greenhouse has an automatic computer-based monitoring and control system for its heater. A temperature sensor continuously measures the air temperature inside the greenhouse. A target temperature is stored in the system's memory. If the measured temperature is below the target, the system switches a heater on; once the measured temperature reaches the target, the system switches the heater off. This checking and adjusting happens continuously, without any human involvement.
(a) The temperature sensor produces an analogue signal, but the system's processor can only work with digital data. Name the hardware component that converts the sensor's analogue signal into digital data, and give its usual abbreviation. [2]
(b) Describe, in order, the steps this system carries out, starting from taking a temperature reading and ending with a decision about the heater. [4]
(c) Explain why this greenhouse system is described as using "feedback". [2]
(d) State whether the heater should be described as an input device or an output device in this system, and justify your answer. [1]
Question 9
A logic circuit has two inputs, D and E, and one output, Y. It is built from three gates, connected as follows:
- Gate 1 is a two-input AND gate, with inputs D and E. Its output is called O1.
- Gate 2 is a two-input NOR gate, with inputs D and E. Its output is called O2.
- Gate 3 is a two-input OR gate, with inputs O1 and O2. Its output is the output of the whole circuit, Y.
Write Boolean expressions in plain text: use . for AND, + for OR, and NOT immediately before
a term for NOT (for example, NOT (D+E)).
(a) Write the Boolean expression for O1 and the Boolean expression for O2, each in terms of D and E. [2]
(b) Write the Boolean expression for Y in terms of O1 and O2, then substitute your answers from part (a) to give Y in terms of D and E. [2]
(c) Complete the truth table for this circuit for all four combinations of D and E, showing the intermediate outputs O1 and O2 as well as the final output Y. [4]
(d) Using your completed truth table, describe in words the relationship between D and E that must hold for Y to equal 1. [1]
Question 10
A logic circuit has four separate binary inputs. A truth table is to be drawn showing the circuit's output for every possible combination of these four inputs. Not counting the header row, how many rows of input combinations does the completed truth table need?